Avoid memcpys in regcache read_part/write_part for full registers.

Additionally, tidy up the functions: Remove asserts, use gdb_byte,
update comments.

gdb/
	* regcache.c (readable_regcache::read_part): Avoid memcpy when
	possible.
	(regcache::write_part): Likewise.
	(readable_regcache::cooked_read_part): Update comment.
	(readable_regcache::cooked_write_part): Likewise.
	* regcache.h: (readable_regcache::read_part): Likewise.
	(regcache::write_part): Likewise.